This invention relates generally to rotatable turbine blades, and more particularly to a method and apparatus installing free standing turbine blades which minimizes problems of galling caused during installation and removal of the turbine blades from their respective rotor.
In steam turbines, arrays of rotating blades are often joined together at their tips by a shroud ring which is normally riveted to the blades via tenons made integral with the blades. Since the tenon is an abrupt change in the cross-section of the blade, it is subject to stress cracking together with bending moments imposed by the shroud ring. It also provides crevices wherein corrosion products can accumulate, often resulting in corrosion cracking. The shroud rings nevertheless greatly reduce blade vibration or "fretting" at the blade root bearing surfaces.
As an alternative arrangement which avoids the necessity of joining the rotating blades at their tips, "free standing" turbine blades have also been used. In such an arrangement, the turbine blades are inserted into serrations formed around the outer periphery of the turbine rotor, and are wedged into place with appropriate combinations of shim stock. One prior art method of installing free standing turbine blades, referred to as the "kicker shim" method, involves the selection of one or more sizes of shims to fill the gap between the a locking device fitted to the root portion of the turbine blade and the bottom of the serration formed in the turbine rotor.
The filler shims selected are first installed at the bottom of the serration, and the turbine blade along with its associated locking device is reinstalled thereover by hand. If more or less filler shims are required, adjustments are made until the turbine blade and locking device can be installed a predetermined distance within the serration. Thereafter, a kicker shim covering only a portion of the length of the serration is installed under all of the other shims, and the blade is tapped into place with a hammer.
One major problem which often occurs with the above-described "kicker shim" method is that it causes the turbine blade roots to be galled or gouged during installation. Such galling is typically due to the necessity of impacting the blade with a hammer after the filler shim pack is tight. Moreover, the shim stock used can become jammed within the serration by deposits precipitated from the steam during flow across the turbine blades, thereby making removal of the blades during maintenance difficult.
It would, therefore, be desirable to employ a method for installing free standing turbine blades upon a rotor which minimizes galling and prevents blade fretting.